I've found different feet work differently. Try a foot that matches it's original, and more than one if necessary. It's my belief, and I will stand corrected if wrong, feet working with feed dogs adjusted correctly should sew a straight stitch whether treadle, handcrank or electric powered.
Edit-I might add, I use a 201, 127, 66, 115, to get best straight stitches. Also, worn dogs or foot (as above) might compromise stitches.
Last edited by Vridar; 10-04-2015 at 01:50 PM.